Evaluation of novel post weld heat treatment in ferritic steel repair welds based on neutron diffraction

The occurrence of cracks in normally welded components with safety relevance in, e.g. nuclear installations or in the petro chemical industry, is not an unusual event. In most cases such cracking is detected in periodic inspections prior to complete failure of the component. Sometimes a dected defect necessitates repair of the damaged component to facilitate its further operatio

NET TG1 Residual stress assessment by neutron diffraction and finite element modeling on a single bead weld on a steel plate

In the context of the efforts of Task Group 1 TG1 of the European Network on Neutron Techniques Standardization for Structural Integrity NET , the Joint Research Centre JRC participated in the experimental round robin campaign for residual stress analysis on a single weld bead on a steel plate. In parallel, the University of Patras UP , in collaboration with the JRC, contributed to the corresponding numerical analysis round robin exercise. Neutron diffraction measurements were performed on a specimen, designated as A12, using the residual stress diffractometer at beam tube HB5 at the High Flux Reactor HFR in Petten, The Netherlands. Several line scans of strains and stresses were performed in accordance with an experimental protocol devised for this exercise and their results are presented in this paper. Two scans were made along the weld longitudinal direction beneath the upper surface of the plate, three were made in the weld transverse direction, and three through the thickness of the plate. The measured residual stresses are presented in detail. The measurements confirm that the stress distribution around this single weld bead on a plate is intrinsically 3 dimensional. The procedure followed by UP in the numerical assessment of the problem is presented in detail. The numerical results are presented in direct comparison to the JRC measurement dat

Residual stress analysis by means of neutron diffraction at research reactors facilities and applications at the HFR

Neutron diffraction is among the scientific techniques available at nuclear research reactors. A particular application of neutron diffraction is the analysis of residual stresses in crystalline materials. The basic principles of the method are described and the unique capabilities of the method are pointed out. The High Flux Reactor HFR is one of about 20 reactor based neutron sources worldwide, where instruments for this technique are available. The HFR equipment is shown and several examples of studies performed during the last decade are given. These examples include nuclear applications, investigations in automotive components and a round robin exercise executed in the context of pre normative research on the method. The examples demonstrate that the method is also suitable for validation of computational analyses or other experimental methods. A comparison between time of flight instruments and monochromatic instruments and an outlook to future developments are given at the en

Residual stress analyses by neutron diffraction in irradiated double V butt welded steel plates

Neutron irradiation is known to have a considerable impact on the mechanical characteristics and the behaviour of materials and components. The distribution of residual stresses is one of the properties affected by irradiation. However, because of the difficulties in performing measurements in radioactive components, not many experimental data have to date been collected. At the High Flux Reactor HFR of the European Commission s Joint Research Centre JRC , a facility has been developed for residual stress measurements in steel specimens subjected to longterm irradiation. The objective of this development was to establish the neutron radiation induced changes in the residual stresses around welds in test pieces representative of the core shroud of boiling water reactors. Residual Stress measurements on such double V butt welds in stainless steel plates after irradiation exposure have been performed by neutron diffraction using this facility. The comparison with measurements in non irradiated companion specimens [1] showed that irradiation changed the distribution of residual stresses. The results suggested that the impact of irradiation varied with the distance of the test location from the specimen surface. On the basis of currently measured data we could not draw definitive conclusion about the influence of neutron dose and irradiation duration upon the stress modificatio

Problems in the averaging of neutron diffraction stress data from round robin campaigns

In recent years the use of a special Bayesian approach on averaging round robin residual stress data has been implemented. This averaging approach is useful in that it copes with the situation where systematic errors have occurred in one or more of the measurements and thus diminishes the influence of these particular wrong value outlier data points. The analyses not only take into account the measurand value, but also the uncertainties associated with each measurand. It should deal with data that may contain individual members with uncertainties larger than the stated error and assumes that the quoted error bar is only a lower bound on the uncertainty. This work shows what could happen when there is a strong mismatch in uncertainties when averaging over a limited amount of data. It has been observed that in a case where there are few data points for example 5 or less , a strong bias can occur towards data points with a relatively small quoted uncertainty compared to other data points with larger quoted uncertainties. A mismatch in uncertainty quotation can arise when averaging very good data with poorer data or when averaging with data obtained from other measurement techniques. This effect is demonstrated in this work by using fictitious data and also based on the example of real measurement data obtained by neutron diffractio

Letterbox type repair weld finite element simulation and residual stress prediction

Finite element prediction of residual stresses in a 3 bead letterbox type repair weld is investigated in the present study. The repair is performed on a 2 CrMo low alloy ferritic steel plate, containing a machined central groove where three weld beads are deposited using AL CROMO S 225 2 CrMo electrodes. The proposed simulation procedure, which is based on decoupled thermal and mechanical analyses and the birth and death of elements technique, is evaluated through comparison of predicted stresses with neutron diffraction testing data. Parametric studies include modelling aspects such as 2 D plane strain versus 3 D analysis, re melting of weld material during sequential bead deposition, melting of base plate near the fusion line and annealing. It is concluded that numerical results come, in general, in satisfied agreement with the experimental dat

Residual strain measurement of C C SiC tubes at high temperature

As part of the European project high and ultrahigh temperature heat exchangers HITHEX the prediction and experimental assessment of the lifetime behaviour, characterisation and qualification of particular CMC materials, including carbon fibre reinforced carbonsiliconcarbides C C SiC , has been executed. Part of the programme of the HITHEX project was the measurement of the strain development within the C C SiC tubular specimens from room to high temperature, the results of which are presented here. Residual strains have been determined in several specimens by neutron diffraction at the High Flux Reactor HFR of the Joint Research Centre in Petten, The Netherlands. At the HFR two facilities are available for residual strain investigations. Both instruments were utilised in the investigations. The first facility at beam tube HB5, the combined stress and powder diffractometer, employs a constant neutron wavelength of 0.257 nm, and the second facility at HB4, the Large Component Neutron diffraction facility, LCNDF, has a flexible wavelength. The installation of a vacuum furnace has enabled the residual strain measurement of specimens at high temperature on HB4. The furnace had to fulfil three main criteria for the investigation of these specimens; high temperature, good neutron penetration and negligible oxidation of the specimens. The ceramic specimens, which have outer and inner diameters of 50 and 40 mm, respectively, and a length of 100 mm have been measured to temperatures of up to 1450 C. Measurements were carried out in two directions on the SiC phase of several specimens, i.e. in the radial and tangential hoop directions. The implications of these results with respect to the structural integrity assessment of these components at high temperatures are discusse

Statistical analysis of residual stress determinations using neutron diffraction

Within Task Group 1 TG1 of the European Network on Neutron Techniques standardization for Structural Integrity NeT , residual stress determinations by neutron diffraction were performed on a single weld bead on a plate BoP specimen by several laboratories. By following a pre defined protocol, residual stress distributions have been estimated in the plate. Each residual strain stress instrument at each institute have their own characteristics and their own methods by which the data is analyzed; however, the final estimate of the stress distributions are expected to be the same to within the quoted uncertainty. This implies that this quoted uncertainty for each stress determination should be realistic. For specific measurement locations in the BoP specimen a modified Bayesian approach was used to obtain an average stress. This special approach is less affected by outliers, although care must be taken as it has been observed that in the case where there are few data points in the average, a strong bias can occur towards data points with a relatively small quoted uncertainty. In the data analyses used here, a fairer average less influenced by such bias is implemented. Presented here is an overview of the main features that influence the stress and associated uncertainty determination, such as reference values, choice of moduli values, grain size effects and positioning et